A light box is a very popular lighting fixture in urban areas. Currently, two types of LED light box are available. The first one is a back-lit light box, see FIG. 1; it has multiple LED sources distributed over the back plate, so that sufficient light can be ensured. However, a bigger space and more LED sources are needed for operation of the back-lit type; this type of light box occupies too much space and consumes a lot of energy. The other one is an edge-lit light box, see FIG. 2; as its name implies, the LED sources are set at the sides, and a reflecting layer is provided at the back to reflect light from the LED sources. Even if the edge-lit type can be made thinner, the light emitted from the LED sources cannot extend far enough if the box is too big. The negative consequence is that the closer to the central part of the light box, the darker the light effect is, which in turn causes non-uniformity.                GB2209422A discloses an arrangement for a LCD display.        US2006/050200A1 discloses a prism sheet to be used as a display device.        